1. Field of the Invention
The present invention relates to the field of semiconductor photodetectors, and in particular to high performance semiconductor back-illuminated photodiode array structures and methods of fabricating the same.
2. Prior Art
Large-scale, multi-element pin and avalanche photodiode arrays have found their application in many imaging applications. Historically, pin photodiode arrays were first developed as front-side illuminated photodetectors with standard die attach and wire bonding processes. However, growing demands of the larger pixel density and smaller “dead” spaces (gaps) between active elements facilitated design of back-side or back illuminated flip-chip structures with virtually unlimited numbers of pixels. See, for instance, S. E. Holland, N. W. Wang, W. W. Moses, “Development of low noise, back-side illuminated silicon photodiode arrays, IEEE TRANS NUC SCI, 44, 443-447, 1997, R. Luhta, R. Mattson, N. Taneja, P. Bui and R. Vasbo, “Back Illuminated Photodiodes for Multislice CT”, Proc. SPIE 5030, 235-245, 2003, U.S. Pat. No. 6,426,991 and U.S. Pat. No. 6,707,046, and U.S. patent application Publication No. 2003/0209652.
In addition to the obvious advantages of large pixel count and small gaps between pixel active areas, conventional back illuminated photodiode arrays have several drawbacks. In particular, in applications with a low or zero reverse bias, the diffusion term dominates the drift in the non-equilibrium carrier collection, thus producing a noticeable, uncontrolled lateral flow of carriers from the illuminated cell (pixel) to the adjacent cells. This effect might be negligibly small for the front-side illuminated arrays, at least within the wavelength range of the small absorption lengths. In the case of back illuminated arrays, the carriers' lateral diffusion results in a significant electrical crosstalk between pixels (>1%), dramatically deteriorating the arrays' performance.
The carriers' diffusion between adjacent cells could be avoided by etching trenches between active elements. However, this method weakens the mechanical integrity of dies, creating additional problems in designing and fabricating thin photodiode arrays.
An alternative solution proposed recently suggests building the structure with isolated diffusion walls between active elements that span the whole thickness of the die. See, for instance, U.S. Pat. No. 6,762,473 and R. A. Metzler, A. O. Goushcha, C. Hicks, and E. Bartley. Ultra-thin, two dimensional, multi-element Si pin photodiode array for multipurpose applications. In: Semiconductor Photodetectors 2004, Proceedings of SPIE, 5353 (SPIE Bellingham, Wash., 2004), 117-125. Such an approach makes the crosstalk negligible (<0.01%), but requires a considerable thermal budget to make a through isolating diffusion between the front side and the backside of the wafer.
Conventional photodiode array structures are based on either the front illuminated or back illuminated ideology. FIG. 1 illustrates a simplified front illuminated photodiode arrays cross-section. Note that the oxide layers are not shown in this Figure. On a substrate 1, either n-type or p-type material, the opposite polarity diffusion 2 is made, creating thereby either p-on-n (p-i-n) or n-on-p (n-i-p) structure, respectively. The anode metal pads 3 for the p-on-n structure (the cathode contacts for the n-on-p structure) are always on the device front surface. The opposite polarity electrode is usually deposited on the chip backside in the case of the front illuminated structure (see 4, FIG. 1). The blanket-type doping 5 on the back surface of the die with the dopant of the same polarity as the majority carriers of the substrate crystal is applied to improve both the charge collection efficiency and DC/AC electrical performance of the devices.
For a back-illuminated structure, the simple design shown in the cross section of FIG. 2 is widely used, with the thickness of the wafer of the first conductivity type anywhere from ˜10 um to ˜500 um. The p/n junction is formed by the shallow diffusion of the dopant of the second conductivity type (see 2 in FIG. 2), and corresponding electrodes are fabricated on the top (see 3 in FIG. 2). The shallow diffusion 6 is made with the dopant of the first conductivity type to provide a good contact to the substrate bulk and to decrease the crosstalk between the adjacent elements. Dummy cell diffusions of the same polarity as the diffusion 2 in FIG. 2 can be placed between active cells to decrease the crosstalk.
Note that similar structures (like diffusion 6 in FIG. 2 and additional dummy cells) are usually used to improve the crosstalk and other performance parameters for the front-illuminated photodiode arrays also.
The back-illuminated photodiode array design proposed recently is characterized with almost zero electrical crosstalk between adjacent pixels due to the isolation diffusion walls that span the whole substrate between the front- and backside of the die (see 7 in FIG. 3). See also, U.S. Pat. No. 6,762,473, R. A. Metzler, A. O. Goushcha, C. Hicks, and E. Bartley. Ultra-thin, two dimensional, multi-element Si pin photodiode array for multipurpose applications. In: Semiconductor Photodetectors 2004, Proceedings of SPIE, 5353 (SPIE Bellingham, Wash., 2004), 117-125, and Tabbert, B., Hicks, C., Bartley, E., Wu, H., Goushcha, I., Metzler, R. A., and Goushcha, A. O. The structure and physical properties of ultra-thin, multi-element Si pin photodiode arrays for medical imaging applications. In: Medical Imaging 2005: Physics of Medical Imaging, Proceedings of SPIE, 5745 (SPIE Bellingham, Wash., 2005), 1146-1154. Note that this isolation diffusion is made of the dopant of the conductivity type opposite to that of the diffusion 2 in FIG. 3, i.e., the same conductivity type as the substrate.